Michelle Williams never moved on after losing Heath Ledger four years ago — until she fell for Jason Segel. After years of platonic friendship, the two have turned up the heat, meeting up for several intimate dates in New York and L.A. Says a Segel source, “Romantically, this kind of happened overnight!” Says another source, “Jason is supersweet. He makes Michelle laugh and feel good about herself.”

On the surface, the demure mom and the freewheeling bachelor seem like a head-scratching coupling, but a Segel insider says, “He has made an effort to clean up his act. Jason has been trying to be more of a grown-up lately. He is taking care of himself more, exercising, eating right and dressing better. Basically, not acting like a frat boy anymore.” According to another Segel source, “He’s super into Michelle, and it just really seems like he doesn’t want to mess this up.”

And Williams could use a true romance. She and Ledger split in 2007 — four and a half months before he overdosed on a combination of prescription drugs — largely because of his escalating drug use, per sources. Still, his passing “took a huge toll on her,” says a Ledger friend. “It was an extremely dark period in her life. She is a romantic and always felt partly responsible for not being able to help Heath. It’s weighed on her a lot.” But while Williams is basking in this new romance, can the pairing last? “Jason can be a bit of a playboy,” says a Segel source. “The only way this will work is if he completely devotes himself to Michelle.” For now, Williams is choosing to remain cautiously optimistic about the Five-Year Engagement star. “She doesn’t want to repeat the same mistakes she made with Heath,” says the Ledger source. “She wants a good guy who will give her a stable family life.”

ADAM LEVINE: Blindsided by the Split

Adam Levine is, well, in misery. After his Sports Illustrated swimsuit-model girlfriend of two years, Anne Vyalitsyna, dumped him, she announced it to the world April 2 without giving him a heads-up. “It was a cheap shot,” says a pal. “Anne didn’t want it to look like he broke up with her.” The Maroon 5 crooner, 33 — who was home in L.A. when she broke up with him from NYC — “was blindsided,” says a Levine friend. “Adam’s heartbroken. He wasn’t even sure they were really broken up.” The insider adds that as recently as late March, “he thought they were together!”

Though the Voice star was leveled by the surprise, friends of the couple saw the split coming. “Anne said he didn’t pay her enough attention,” says a pal of the model, 26, who hung out in Miami in late March while Levine worked in Costa Rica. Another gripe: He wouldn’t put a ring on it. “As a woman, you get to a point where you want to get married,” adds an insider. “Adam wasn’t ready to take that step.” Vyalitsyna, it seems, is now on the hunt for someone who will. Says a source of the model — seen partying at NYC’s Darby April 1 — “She’s telling people she wants to meet guys!”